1. Field of the Invention
The present invention relates to a facing-targets-type sputtering apparatus including a sputtering unit, in which a pair of facing targets are disposed a predetermined distance away from each other, and permanent magnets are disposed along the periphery of each facing target such that magnetic poles of one target face the corresponding magnetic poles of the other target. The sputtering apparatus is configured such that a magnetic field extending in the direction perpendicular to the facing targets is generated in such a manner as to surround a space provided between the facing targets (the space is hereinafter called a “confinement space”) so as to confine plasma within the confinement space and to perform sputtering of the targets, thereby forming a film on a substrate disposed beside the confinement space.
2. Description of the Related Art
A facing-targets-type sputtering apparatus is disclosed in Japanese Patent Publication (kokoku) Nos. 63-20303, 63-20304, and 62-14633 previously filed by the present inventors. The sputtering apparatus have a basic structure shown in FIG. 1 (Prior Art). Specifically, the apparatus includes a sputtering unit including targets 110a and 110b, and permanent magnets 130a and 130b serving as magnetic-field generation means. The targets 110a and 110b are disposed a predetermined distance away from each other within a vacuum chamber 10, thereby providing a confinement space 120 therebetween. The permanent magnets 130a and 130b are disposed behind the corresponding targets 110a and 110b in order to generate a magnetic field which extends in the direction extending between the targets 11a and 110b and which uniformly surrounds the confinement space 120. A substrate holder 21 disposed beside the confinement space 120 holds a substrate 20 such that the substrate 20 faces the confinement space 120. In FIG. 1, reference numeral 11 denotes a chamber wall of the vacuum chamber 10, and reference numerals 140a and 140b denote shields for protecting, from sputtering, portions of target units 100a and 100b other than the front surfaces of the targets 111a and 110b. 
After the vacuum chamber 10 is evacuated through an evacuation port 30 by means of an unillustrated evacuation system, a sputtering gas, such as argon, is introduced into the vacuum chamber 10 through a gas inlet 40 by means of unillustrated gas introduction means so as to attain a predetermined pressure. As shown in FIG. 1, a DC sputtering power supply 50 supplies sputtering power to the apparatus while the shields 140a and 140b; i.e., the vacuum chamber vessel 10, serve as an anode (ground) and the targets 110a and 110b serve as a cathode. Thus, sputtering plasma is generated within the confinement space 120, and the sputtering plasma effects sputtering of the targets 110a and 110b, thereby forming on the substrate 20 a thin film whose composition corresponds to that of the targets 110a and 110b. Depending on the composition of the targets, the sputtering power supply may be a high-frequency power supply.
Since the magnetic field extends in the direction perpendicular to the targets 110a and 110b, high-energy electrons are confined within the confinement space 120, and plasma is generated therein. The plasma accelerates ionization of the sputtering gas, thereby increasing a sputtering rate and thus forming a film at high rate. Unlike the case of a planar-magnetron-type sputtering apparatus (typical conventional sputtering apparatus) in which a substrate is disposed so as to face a target, the substrate 20 is disposed beside the targets 110a and 110b. Therefore, ions and electrons which impinge on the substrate 20 are greatly reduced, and thermal radiation from the targets 110a and 110b to the substrate 20 is reduced, thereby suppressing an increase in substrate temperature. Thus, a film can be formed at low temperature. Unlike the case of such a conventional magnetron-type sputtering apparatus, which encounters difficulty in forming a film of magnetic material at high rate, the facing-targets-type sputtering apparatus enables formation of films of various materials including magnetic material at low temperature and at high rate, and has thus been employed for producing a variety of thin films, including magnetic thin films for thin-film-type magnetic recording media and magnetic heads, metallic films, metal oxide films, and ceramic films.
The facing-targets-type sputtering apparatus generally employs rectangular or circular targets. However, regardless of target shape, a target tends to be sputtered intensively at a central portion of the target surface, indicating the necessity to improve target utilization efficiency. When a rectangular target extending in a width direction of a substrate is employed, a target erosion pattern becomes asymmetrical with respect to the center of the target. As a result, variation in film thickness arises in a width direction of the substrate, indicating the necessity to improve productivity and uniformity of film thickness.
In order to solve the aforementioned problems, the present inventors have proposed, in Japanese Patent Publication (kokoku) No. 5-75827, a technique for improving a feature of a facing-targets-type sputtering method; i.e., a technique for attaining confinement of plasma over the entire surface of a target under uniform conditions. In order to generate and confine sputtering plasma, the proposed technique employs electron reflection means for reflecting electrons toward a space in the vicinity of a peripheral edge portion of a target, in addition to magnetic-field generation means for generating a magnetic field in the direction perpendicular to the target surface, the magnetic-field generation means being provided along the peripheral edge portion of the target and employed in a conventional facing-targets-type sputtering method. In a sputtering apparatus employing this technique, in addition to a magnetic field in the direction perpendicular to the target (i.e., a facing-mode magnetic field), a magnetron-mode magnetic field, which is generated in a conventional planar-magnetron sputtering apparatus, is generated in the vicinity of the peripheral edge portion of the target, and extends from the electron reflection means toward the center portion of the target. Therefore, high-energy electrons drift by means of the facing-mode magnetic field within a space provided between a pair of facing targets, and drift by means of the magnetron-mode magnetic field over the peripheral edge portion of each of the targets without being absorbed by magnetic poles. Thus, the ionization efficiency of a sputtering gas is significantly enhanced, and sputtering efficiency is enhanced over the entire surface of the targets. This sputtering technique enables formation of a thin film of very fine structure with excellent characteristics, as compared with a case of a conventional sputtering method in which a substrate and a sputtering source face each other and which cannot form a thin film in such a manner. Also, the technique permits almost uniform erosion of the target over its entire surface. Therefore, even when sputtering of a rectangular target is performed, the symmetry of a target erosion pattern with respect to a central portion of the target is improved.
Industrialization of such a sputtering apparatus requires improvements to productivity and long-term reliability. In order to meet such requirements, the present inventors have proposed, in Japanese Patent Application Laid-Open (kokai) No. 10-330936, a facing-targets-type sputtering apparatus including a target support unit having a cooling section and a section for accommodating a permanent magnetic serving as magnetic-field generation means, in which the permanent magnet is separated from a vacuum chamber and can be cooled. Through use of the sputtering apparatus, a film of high quality can be formed. In addition, power applied to the apparatus can be increased greatly, and performance of the permanent magnet can be maintained over a long period of time. FIG. 2 shows distribution of the magnetic fluxes of magnetic fields in the sputtering apparatus disclosed in Japanese Patent Publication (kokoku) No. 5-75827 or Japanese Patent Application Laid-Open (kokai) No. 10-330936.
Meanwhile, industrialization of the aforementioned facing-targets-type sputtering apparatus has been found to require the below-described improvements.
When, by use of the aforementioned conventional facing-targets-type sputtering apparatus, a film is formed on a substrate having a high width/length ratio or continuous film formation is performed while a substrate is conveyed at a predetermined rate, like the case where continuous film formation is performed through other physical deposition methods, variation in film thickness arises in a width direction of the substrate (as used herein, the expression “width direction” refers to the direction perpendicular to the sheet in FIG. 1). The aforementioned conventional facing-targets-type sputtering apparatus includes no means for regulating such variation in film thickness in the width direction, and the degree of variation in film thickness in the width direction depends on the size of a target. Therefore, in order to reduce variation in the thickness of a film to a predetermined level, a target having a long width must be employed. Thus, a large-sized apparatus is required, and utilization efficiency of a target is lowered, resulting in high production costs. Therefore, industrialization of the aforementioned sputtering apparatus requires improvements in relation to production costs.
Meanwhile, when formation of a thin film is performed by use of a rectangular target of a non-magnetic material in the aforementioned sputtering apparatus over a long period of time, although the entire surface of the target is almost uniformly eroded, the degree of erosion of a first pair of diagonal corners of the target somewhat differs from that of a second pair of diagonal corners of the target. Therefore, industrialization of the sputtering apparatus has been found to require improvements in terms of utilization efficiency of a target and long-term operation of the apparatus.
As the aforementioned facing-targets-type sputtering apparatus is employed in a variety of fields, the below-described demands in terms of film formation have arisen for the sputtering apparatus.
(1) Realization of formation of a functional film at a low temperature while suppressing heating of a substrate. This demand arises particularly when a plastic substrate is employed or a functional film is formed on an organic underlying layer.
(2) Realization of formation of a functional film on an underlying layer while suppressing the effects of plasma, ions, electrons, etc. on the underlying layer so as to protect the layer from any damage.
(3) Realization of sputtering under high vacuum for formation of a high-quality film.
Industrialization of the aforementioned sputtering apparatus requires further improvements in terms of reliability, maintenance, and equipment cost. Since the sputtering apparatus has a structure in which a cooling jacket is provided at the interface between a support unit and a backing unit of the target, when the apparatus is operated for a long period of time, deterioration of a vacuum seal occurs with passage of time. Therefore, such a problem has remained unsolved.